Here's what happens to your body when something terrifies you

Universal
You freeze, your heart feels like it might beat out of your chest, your legs shake, and the hair on the back of your neck stands on end.

It's a familiar feeling that can be triggered by anything from a scary movie to someone jumping out and yelling "Boo!" But why does the body react this way when we get scared?

In honor of Halloween, we decided to find out.

Fear starts with a trigger. When something frightening happens, like someone jumping out at you in a haunted house, that's a stimulus that signals to your brain that you might be in danger.

The amygdala is deep and ancient region of the brain.Life Science Databases (LSDB)/Wikimedia (CC BY-SA 2.1)
Whether the stimulus is touch, sight, or sound, the scary signal quickly reaches the thalamus at the center of the brain and travels down to the amygdala, at the base of the brain.

From here a neurotransmitter called glutamate then carries the signal even deeper into the brain, Abigail Marsh, a psychology researcher at Georgetown University, said in BytesizeScience video. This causes us to freeze or involuntarily jump — the "fight-or-flight" response.

These two reactions are automatic and involuntary because the deep brain is ancient in terms of evolution, according to Marsh, and we have little control over it. The reason is because a fight-or-flight response unleashes powerful hormones that affect the entire body.

A model of an adrenaline, also known as epinephrine.Jynto/Public Domain
The hormonal surge also causes your heart to pump blood more forcefully to the muscles. That's why you might feel a little shaky or unsteady when you're scared — the extra blood is getting your body ready to sprint away from the danger or stand and fight if you need to, according to the California Science Institute.

The fight-or-flight system was pretty useful for early humans, who regularly faced off against giant beasts (and each other), but modern society doesn't have as much need for it these days. In fact, the fight-or-flight response can be lethal in modern times: We now live to such old ages that getting scared can trigger a heart attack, neurologist Martin Samuels told Scientific American.

Our bodies can reverse the fear response fairly quickly, though. If it turns out we aren't in a life-threatening situation, the parasympathetic nervous system (PSNS) kicks in to counter the fight-or-flight instinct, primarily by stopping the flood of adrenaline and lowering our heart rate back to normal.

That's why every time we jump during a scary movie we don't run out of the theater screaming; after the initial reaction, our PSNS helps us recognize the threat is not real and calms us down.

Part of the reason the PSNS exists is because adrenaline can be toxic in large amounts. If too much adrenaline floods into the heart, it can lead to the failure of that organ and death, emergency physician Rober Glatter told Live Science.

And it's not just fear that's a threat: Any extreme emotion can trigger a flood of adrenaline. You can't manually stop the fight-or-flight response from happening, but things like practicing meditation might help you stay more calm the next time you do grapple with terror or other powerful emotions, Glatter told Live Science.